Freezing Point Osmometer

Freezing Point Osmometer.

KNAUER is one of the pioneers in the field of osmometry and known for its reliable and user-friendly instruments for many decades. KNAUER‘s newest freezing point osmometer K-7400S allows the easy and fast determination of the osmolality of various aqueous solutions. Also, the freezing point depression of the samples can be read.

The proven technology of freezing point determination in combination with the robust and intelligent design of the device allows fast and reproducible measurements. The instrument is equipped with a peltier cooler and an integrated microprocessor controlling the automated measurement.


Food Analysis

For analysis and quality control of milk or beverages like isotonic drinks and beer

Pharmaceutical Analysis

For quality control of infusion solutions, inhalation, rinsing solutions and eye drops.


For research on plant sap and insect haemolymph and testing of nutrient solution and cell culture media.


For process monitoring and quality checks of reagents and chemicals.


Theory of osmolality

Osmolality is a general measure of the particle concentration in a solute. It´s not depending on the nature of molecules but just their number. Therefore, a two molar solution of a non-dissociating molecule (A) has the same osmolality as a one molar solution of a fully dissociating salt composed of two ions (B). The osmolality of a solution is the same, even when molecules vary in shape (C) as well as in size (D). Therefore, all solutions containing the same number of osmotically active particles — regardless of their chemical properties — exhibit the same osmolality.


Freezing point osmometry

The measurement principle of the K-7400S Semi-Micro Osmometer is based on the colligative property of freezing point depression. If a solute is added to a liquid this results in a decreased freezing point of the solution. The depression is 1.858 K per 1 mole of ideally solved compound in one liter of water. This effect depends only on the number of particles in the liquid and not on the physical or chemical properties of the solutes. Due to this linear correlation, the osmolality of a sample can be determined by precisely measuring its freezing point.



At the beginning of a measurement the sample is cooled by a microprocessor-controlled peltier element. During this process, the solution is supercooled below 0 °C while still being liquid. At a certain temperature the freezing process is initiated by a rotation of the stirring wire. The formation of ice crystals causes the release of thermal energy, thus rising the temperature of the sample. After a short period of time an equilibrium is reached where melting and thawing of ice crystals are balanced and the sample’s temperature stays constant. This plateau marks the real freezing point of the sample. During the whole process the temperature of the solution is measured by a high-precision thermistor. Thanks to a resolution of 1/1000 K the freezing point temperature is exactly determined and even small differences in osmolality of two samples can be measured.


Quality control of pharmaceutical
solutions by determination of osmolality

The osmolality of solutions used in the clinical and pharmaceutical field is an important issue that requires regular monitoring. Especially infusion solutions but also solutions for external use like eye drops and rinsing solutions must be isotonic to ensure the physical well-being of humans. In this study, the osmolality of commercially available pharmaceutical solutions were determined using the KNAUER K-7400S Semi-Micro Osmometer.


Determination of osmolality of isotonic and non-isotonic beverages

In the last years isotonic beverages have become more and more popular. Almost every drink is available in an isotonic version. But what does isotonic mean and what are its benefits? How can you check that beverages are really isotonic? And how is osmolality involved?

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